A liposome is a roughly spherical hollow particle enclosed by at least one lipid bilayer composed of lipid molecules. The lipid molecule possesses hydrophilic groups having hydrophilicity and, on the opposite side, lipophilic groups having lipophilicity. For this reason, in contact with water, the molecules form a bilayer, which becomes spherical so that the surface area is minimal, the hydrophilic groups facing outside the bilayer as well as inward toward the inner compartment formed by the bilayer, while the lipophilic groups pointing toward the interior of the bilayer. Since the bilayer thus forming a liposome is similar to the cell membrane constituting the living body, it is easily accepted in the biological environment. In recent years, by taking advantage of this property, liposomes have been a focus of attention as pharmaceutical vesicles in the drug delivery system (DDS), in which a drug encapsulated in the region surrounded by the bilayer is transported to the site in the living body requiring the medicine.
Various techniques have been known as the methods for producing liposomes, of which one typical technique is the Bangham method (thin-film method). In this method, a suspension containing liposomes is obtained as follows: at least one phospholipid is dissolved in an organic solvent, such as chloroform, in a vessel, such as a flask; then by evaporating off chloroform, lipid membrane is temporarily formed at the bottom of the vessel, to which an aqueous solution, such as buffer, is added and the vessel is mixed (A. D. Bangham et al., J. Mol. Biol., 13, 238-252 (1965); A. D. Bangham and R. W. Horne, J. Mol. Biol., 8, 660-668 (1964).
Alternatively, typical industrial manufacturing methods for liposomes include the technique in which lipid components, such as phospholipid dissolved in a water-miscible organic solvent, is added to an aqueous solution by infusion with stirring. The water-miscible organic solvents which can be suitably used here include alcohols, such as methanol, ethanol, isopropyl alcohol, and butanol. It should be noted, however, that the lipid solution should be added to and mixed with an aqueous solution while being warmed so that the dissolved state of the lipid is maintained; this requires precise control of temperature, adding speed or stirring speed (National Publication of International Patent Application No. 2006-517594). Furthermore, another method of producing a preliposome using t-butanol has also been reported (Japanese Translation of International Patent Application No. 1994-509547) This method has an advantage that lipid can be sterilized by filtration and be freeze-dried, since lipid can be maintained as a solution around room temperature by using, as a dissolution solvent of lipid, t-butanol having a water content as low as about 20%.